1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method and program for controlling temperature of a fixing unit in fixing a formed toner image on a printing medium.
2. Description of the Related Art
In an image forming apparatus for fixing a toner image formed by electrophotography on a printing medium such as transfer paper, it is necessary to control a fixing temperature in order to reliably fix the toner image. As methods for determining a fixing temperature, various techniques already exist. For example, there is a method in which in order to reliably fixing an image whose amount of toner applied on transfer paper is large, the characteristics of an image (a character, a graph, and a high-density portion having a large area) are identified based on a distribution ratio of an image area value for each image density, and a fixing temperature for fixing an image is determined according to the characteristics of the image (see Japanese Patent Laid-open No. 2000-221831).
In Japanese Patent Laid-open No. 2000-221831, the fixing temperature is controlled according to the distribution ratio of the image area value for each image density, but the continuity of pixels having a predetermined density is not determined. Accordingly, it is impossible to determine whether pixels having a high density are positioned in an image discretely or in a concentrated manner.
In a case where the fixing temperature of a fixing device is determined according to the distribution ratio of the image area value as disclosed in Japanese Patent Laid-open No. 2000-221831, the fixing temperature may fail to be controlled with high precision for the image. A specific explanation thereof will be given below.
A temperature at which an image is fixed is determined according to heat efficiency for the contact area of a transfer sheet at the time of passing through a nip portion of the fixing device. Accordingly, in a case where the fixing temperature of the fixing device is determined according to the distribution ratio of the image area value as disclosed in Japanese Patent Laid-open No. 2000-221831, the image may be fixed at a temperature which is higher or lower than an anticipated temperature. More specifically, in a case where in the image, pixels whose amount of applied toner is equal to or larger than a predetermined threshold are distributed as a large number of portions whose area is smaller than the area of contact between the nip portion of the fixing device and the transfer sheet (i.e., in a case where pixels having a high density are distributed discretely), the image is fixed at a temperature which is higher than the anticipated temperature. Further, conversely, in a case where in the image, pixels whose amount of applied toner is equal to or larger than a predetermined threshold are distributed as a small number of portions whose area is larger than the area of contact between the nip portion of the fixing device and the transfer sheet (i.e., in a case where pixels having a high density are distributed in a concentrated manner), the image is fixed at a temperature which is lower than the anticipated temperature. This causes curling of the transfer sheet which is due to high-temperature control, a failure to fix the image which is due to low-temperature control, or the like.